1. Field of the Invention
The present invention relates to an electric double layer capacitor.
2. Prior Art
To enhance the characteristics of electric double layer capacitors, the practice until now has been to adjust within a suitable range the volume ratio of the amount of electrolytic solution to the void volume of the polarizable electrodes and the separator.
For example, JP No. 3008399, JP-A 2001-196273, JP-A 2002-93663, JP-A 2004-253562 and JP-A 2005-45180 report that electric double layer capacitors of an excellent durability and temperature characteristics and a low resistance can be obtained by controlling to a suitable value the volume ratio of the amount of electrolytic solution to the void volume of the polarizable electrodes and the separator.
The desirability of adding an excess amount of the electrolytic solution to prevent the electrolytic solution within a capacitor from drying up is clearly recognized in the above-cited art.
Yet, merely optimizing the amount of electrolytic solution in the manner indicated in the above art is not sufficient for achieving an electric double layer capacitor with a higher voltage rating.
In JP-A 2000-286161, which focuses on the relationship between the quantity of electricity charged and the amount of ions in a capacitor, a technique is disclosed for controlling the amount of electrolyte to a level where ions are present only in the vicinity of the electrode surfaces when the capacitor is fully charged. JP No. 3389353 discloses an approach which also takes into consideration the rise in internal resistance; the amount of electrolyte in the electrolytic solution is lowered to a level at which the internal resistance rises, and the electrode filling amount is supplemented to a degree that corresponds to this difference. JP-A 2003-257793 discloses an approach that involves regulating the ion concentration wherein, by adjusting the amount of electrolyte, the electrolytic solution becomes a dielectric at voltage settings above the maximum operating voltage of the capacitor and below the decomposition voltage of the electrolytic solution.
However, in the art described in above JP-A 2000-286161, JP No. 3389353 and JP-A 2003-257793, the aim is to achieve a high capacitance even when the internal resistance is allowed to increase, or to prevent decomposition of the electrolytic solution by making it electrically insulating. Hence, limits are placed on the amount of electrolyte.
Therefore, in such art, the energy density of the electric double layer capacitor is higher, yet the internal resistance rises as the capacitor comes closer to being fully charged. Because this leads to a decrease in the charge-discharge efficiency and a reduction in discharge energy during high-current charging and discharging, the result is a decline in the cycle characteristics. Moreover, lowering the electrolyte concentration may lead to a marked increase in the internal resistance within a low-temperature environment, making charging and discharging impossible to carry out.